1. Field of the Invention
The present invention relates to a catalyst based on a noble metal and/or a noble-metal oxide on an aluminum oxide support for the selective hydrogenation of an unsaturated compound, to a process for the preparation of the catalyst and to a process for the selective hydrogenation of unsaturated compounds.
2. Discussion of the Background
Selective hydrogenations on catalysts are well-known in the chemical industry and in the oil-processing industry, where they can have various applications. For example, a selective hydrogenation can be carried out in order to free certain products, such as, for example, ethene or propene, from undesired by-products, such as diolefins and acetylenes, which would interfere with further processing of the olefins. Another application may be the selective hydrogenation of substances which form during a reaction and, as undesired by-products, cause a reduction in the overall yield of the target products. Selective hydrogenation can be used, if desired, to convert such byproducts back into starting materials, which can then be recycled into the process.
A typical example of the latter application is the selective hydrogenation of .alpha.-methylstyrene to cumene in processes for the preparation of phenol and acetone, for example in the Hock phenol synthesis (see Weissermel/Arpe, Industrielle Organische Chemie [Industrial Organic Chemistry], Verlag Chemie, 1976, pp. 291 ff.). A by-product obtained in the phenol synthesis is .alpha.-methylstyrene. Selective hydrogenation of the .alpha.-methylstyrene to cumene gives fresh starting material, which, if recycled into the process, ultimately results in improved efficiency of the process.
One of the crucial factors for selective hydrogenation is the initial selectivity of the catalyst employed. If an excessive amount of by-product(s) is formed at the beginning of the selective hydrogenation (for example, due to undesired total hydrogenation), a prolonged start-up procedure for the hydrogenation plant and a decrease in the yield of the process arise, resulting in a reduction in overall efficiency.
DE-A 2,758,318 discloses a process for the selective hydrogenation of unsaturated compounds in which the hydrogenation catalyst is brought into contact with, inter alia, the components to be hydrogenated for an appropriate length of time in the absence of hydrogen.
DE-A 2,758,274 discloses that the hydrogenation selectivity of a hydrogenation catalyst can be increased by treating the hydrogenation catalyst with gaseous ammonia for a sufficient length of time and subsequently carrying out the selective catalytic hydrogenation of the unsaturated compounds with hydrogen.
The catalysts used are, in particular, noble metals, such as ruthenium, rhodium, palladium or platinum, on supports. Examples of suitable supports are clays, magnesium oxide, gels of silicon dioxide and aluminum oxide, zeolites, activated charcoal and activated aluminum oxide, which may each be in various forms, such as pellets, beads, etc.
Both of the above processes are complex and result in increased operating costs due to the complicated start-up procedures necessary for the catalyst.